Fish investigations in the Barents Sea Winter 2022

Table 1.1 presents the vessels participating in the survey in 2022 and IMR trawl station series numbers, and Figure 1.1 shows survey tracks, trawl stations and ice cover.

The coverage of the most northern and most eastern strata differs from year to year. The areas of these strata are therefore calculated according to the coverage each year. Table 1.3 gives the area covered by the survey every year since 1981. In that table “Extrapolated area” reflects the size of areas where some kind of extrapolations/adjustments have been made to take account of incomplete coverage (see also section 3.1). Table 1.4 summarizes the degree of coverage and main reasons for incomplete coverage in the whole period.

¹ Russian EEZ not covered

² Russian EEZ not completely covered (Strata 7 and 13 in 2006, Area D’ in 2012, strata 7, 13, 15, 7 and 20 in 2017, strata 17, 19, and 20 in 2020, and strata 16, 19, and 20 in 2021).

³ Additional northern areas (N) covered from this year.

Individual lengths are collected from all target species, while otoliths for age determination are taken from cod, haddock, and capelin. For cod and haddock, the otolith readings are key for splitting the survey indices by age.

For both the bottom trawl and acoustic estimates, cod with all otolith types (coastal cod included) are included in the estimations. Coastal cod is usually well under 5% of the total.

Table 2.1 gives an account of the sampled length- and age material from bottom hauls and pelagic hauls from 1994 onwards.

Table 2.2. shows the number of age readings per age for cod from 1994 onwards, while table 2.3 shows the same for haddock. The number of age samples for fish age 10+ increased in the second half of the time series, reflecting changing age composition in the stocks.

Details on the calculation of survey indices, including StoX settings for different species are found in Appendix 1.

In 2022, the swept area and acoustic ¹ estimation in StoX was based on the following biotic and acoustic snapshot files (versioned trawl and acoustic data):  

¹ Acoustic estimation is done for cod and haddock only. The biotic files are used in the acoustic StoX projects to split the acoustic backscatter by age.

3.1 - Raising of indices

In 1997, 1998 and 2007 only the Norwegian exclusive economic zone (EEZ) and parts of the Svalbard area (S) was covered. The swept-area indices for cod, haddock, golden redfish, beaked redfish and Greenland halibut have therefore been raised to also represent the Russian exclusive economic zone (EEZ) (Mehl et al . 2016).

In 2006, there was not complete coverage in the southeast due to restrictions. The observations in the partially covered strata 7 were extrapolated to the full strata, and the observations in the partially covered strata 13 were extrapolated to the same area as covered in 2005.

In 2012 the coverage was incomplete in the eastern areas, and the cod and haddock swept area estimates within the covered area were raised by the “index ratio by age” observed for the same area in 2008-2011 (ICES 2012). The scaling factor (“index ratio”) for estimating adjusted total from <Total – area D’> was the average ratio by age for Total/(Total – area D’) in the years 2008-2011 (Aglen et al. 2012).

In 2017, the Norwegian vessel was not allowed to operate south of 70º 10’ N and west of 41º 00 º E, and no Russian vessel participated in the survey. Only a small part of strata 7 was covered, and strata 13, 15, 17 and 20 were not covered. The cod, haddock, Greenland halibut and beaked redfish swept area estimates and cod and haddock acoustic estimates within the covered area were raised following the same procedure as for 2012. The scaling factor for estimating adjusted total from <Total –strata 7 > was the average ratio by age for Total/(Total – (strata 7+13+15+17+20)) swept area indices in the years 2014-2016.

In 2020, coverage was incomplete in strata 17, 19, and 20, and the cod and haddock acoustic and swept area estimates were raised by the “index ratio by age” observed for these strata in 2018-2019. The scaling factor for estimating adjusted total from <Total – strata 17, 19 and 20> was the average ratio by age for Total/(Total – (strata 17+19+20)) in the years 2018-2019.

In 2021, coverage was incomplete in strata 16, 19, and 20. Indices in the partly covered stratum 19 were extrapolated to the entire strata. No trawling was done in stratum 20. As cod and haddock abundances generally are low there, the stratum was partly ice covered and did not have coverage in the last two years, this stratum was excluded from estimation. Only one trawl station was taken in stratum 16. Here the cod and haddock acoustic and swept area estimates were raised by the “index ratio by age” observed for these strata in 2019-2020. The scaling factor for estimating adjusted total from <Total – strata 16> was the average ratio by age for Total/(Total – strata 16) in the years 2019-2020.

Table 4.1 presents the time series of total echo abundance (mean s_A multiplied by strata area and summed over all strata) of cod and haddock in the investigated areas.

¹ not scaled for uncovered areas

Since 1993 the acoustic values have been split between the two species during the scrutinizing. The values for cod have shown an increasing trend since the late 2000s, with a peak in 2013-2015. Total echo abundance was 40% lower in 2016 compared to 2015 and decreased further from 2016 to 2017, while there was an increase of more than 50% from 2017 to 2018-2019 and then a decrease in 2020 to a level similar to that in 2017. The 2021 echo abundance is the third lowest in the time series, and the lowest observed since 2006 (the lower value in 2007 likely reflects the lack of coverage of the Russian zone and is not directly comparable). The echo abundance increased slightly in 2022, but it is still among the lowest values in the time series.

The values for haddock increased gradually from the end of the 1990s to 2008, decreased gradually to less than one third of the 2008-value in 2016 but increased considerably in 2017 and further in 2018 and 2019, before decreasing in 2020. The 2021 echo abundance is the fourth lowest in the time series, and the lowest observed since 1999. In 2022 the echo abundance increased again, but is still the fourth lowest since 1999.

5.1 - Acoustic estimation

Surveys in the Barents Sea at this time of the year mainly cover the immature part of the cod stock. Most of the mature cod (age 7 and older) have started on their spawning migration southwards out of the investigated area and are therefore to a lesser extent covered. There are indications that a higher proportion than normal spawned along Finnmark in some years, e.g., 2004-2006. Thereby, a higher proportion of spawners might have been covered by the survey in those years. Figure 5.1 shows the spatial distribution of acoustic registrations assigned to cod in 2022. The registrations reflect the general distribution of cod in the central and southwestern Barents Sea. The NASC values in 2022 were low, reflecting the overall low echo abundance.

Table 5.1 shows the acoustic indices for each age group by main areas in 2022. 26 % of the 1-year-olds were found in the extended area (N) in 2022 compared to 7 % in 2021. Age 3 cod had the highest percentage of total abundance in area N with 35 %. The time series of total abundance at age (1994-2022) is presented in Table 5.2.

The acoustic estimates have been variable and increasing in later years, with a peak in biomass in 2013, and this may partly be explained by variable and not complete coverage of the distribution area towards north and east in several years. As cod grow older it gets a more south-westerly distribution during winter, so that it “grows into” the covered area with increasing age. This is especially evident for the strong 2004 and 2005 year-classes, which as 6-11-year-olds stand out as the strongest in the time series. The 2019-2020 year-classes were among the lowest in the time series both at age 1 and 2 while the 2021 year-class was moderate at age 1. Table 5.3 shows time series for strata 24-26 (area N) in 2014-2022, which are included in the main time series.

Table 5.4 presents estimated coefficients of variation (CV) for cod age groups 1-14 in 1994-2022. These estimates were obtained by using StoX with a stratified bootstrap routine treating each transect as the primary sampling unit. In addition, a bootstrap routine for all trawl stations by strata was carried out within each run. The estimated CV (Standard Deviation ∙ 100/mean) is estimated from 500 iterations . A CV of 20% or less could be viewed as acceptable in a traditional stock assessment approach if the indices are unbiased (conditional on a catchability model). In 2022 the age groups 1 and 3-7 fall into this category. Values above this indicate higher uncertainty of the estimated index, with reduced information regarding year-class strength. In all years, CVs for age groups older than 10 years are above what could be considered as acceptable. This is to a large degree related to low catch rates resulting in fewer age samples for these age groups (Table 2.2).